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docs(ui): route the undefined session-id case to the direct seam (RIG-2874) #1097

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Merged
trunk-io merged 1 commit into from
Sep 11, 2026
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docs(ui): route the undefined session-id case to the direct seam (RIG-2874) #1097

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8 changes: 7 additions & 1 deletion docs/designs/ui/compass-outbound-session-header/design.md
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Original file line number Diff line number Diff line change
Expand Up @@ -579,7 +579,13 @@ Cases:

- getter returns a valid id ⇒ request carries `X-POSTHOG-SESSION-ID: <id>`
(capture seam).
- getter returns `undefined` ⇒ header **absent** (not empty) — capture seam.
- getter returns `undefined` ⇒ header **absent** (not empty) — **direct seam,
all three assertions above**. The guard can throw on this input, so absence
alone is not enough. `captureRequest`'s capturing `fetch` always `throw`s
(`index.test.ts:87`), so its gate `rejects.toThrow()` cannot tell "threw
early" from "reached `next`" — it goes green on the very defect the case
exists to catch. Assertion 3 (`result` is the sentinel) is what proves
nothing threw.
- getter returns `""` ⇒ header absent — capture seam.
- oversized value (201 ASCII chars) ⇒ header absent — capture seam.
- **exactly 200 ASCII chars ⇒ header PRESENT** (capture seam). This is the
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